This invention pertains generally to implantable medical devices, and more particularly to implantable medical devices for applying coordinated defibrillation electrical energy to the heart.
Electric shock defibrillation is a proven technique of treatment of the serious and immediately life-threatening condition of ventricular fibrillation. For patients known to be at risk, an implantable defibrillator may be used. Such devices contain an energy source, an electrode lead system in contact in the heart, a sensing system to detect the onset of fibrillation, and a pulse generator for delivering the defibrillation shock.
Existing devices are generally designed or programmed to deliver a shock or series of shocks at a fixed interval or intervals following the detection of the fibrillation, unless fibrillation spontaneously terminates on its own first, or until recovery is achieved, as evidenced by the resumption of normal ventricular rhythm. The amount of energy to be delivered in a shock must be carefully chosen. If too small, it may not be successful in terminating the fibrillation. On the other hand, the shock must not be so large that it causes damage to the myocardium. The device generally is designed in consideration of the limited energy storage in an implanted device.
Ventricular electrical signals may exhibit a pattern, known as xe2x80x9cfine ventricular fibrillationxe2x80x9d during ventricular fibrillation. The fine ventricular fibrillation is characterized by relatively low signal amplitude and lack of organized features. The ventricular electrical signals may also exhibit a pattern known as xe2x80x9ccoarse ventricular fibrillation,xe2x80x9d characterized by intervals of relatively higher amplitude, which may repeat, separated by fine ventricular fibrillation intervals. It has also been suspected that it is easier to defibrillate coarse ventricular fibrillation than fine ventricular fibrillation. Because of this, previous works have suggested the possibility of timing of defibrillation shocks to features of the ventricular fibrillation waveforms as a way to improve defibrillation efficacy. However, it has not been clear from such prior works, which features are important, and how to detect and coordinate to them. A need, therefore, exists in the art for a system that improves defibrillation therapy by using the minimal amount of energy necessary to bring about effective and efficient defibrillation.
The present invention provides an improved defibrillator system. The defibrillator system determines an optimal time for the delivery of defibrillation shocks, such that the shocks delivered have an improved probability of success in terminating the fibrillation. This improved efficacy provides important medical advantages to the patient, both in the greater probability of success of individual shocks, and also in the reduction in pulse energy and number of shocks needed to defibrillate. This in turn will result in a smaller implantable defibrillator that can deliver more shocks over the lifetime of the battery.
The defibrillator system detects characteristics of arrhythmia complexes which exist during ventricular fibrillation of a heart, and coordinates the delivery of ventricular defibrillation shocks with portions of the complexes. In one embodiment, the defibrillator system monitors a first cardiac signal across a first cardiac region. The first cardiac region, in one embodiment, is in a left ventricular cardiac region of the heart. Upon detecting a ventricular fibrillation of the heart, the defibrillator system delivers a defibrillation shock during, or at the termination, of a coupling interval time period. The coupling interval time period is a preprogrammed time which is started once a contraction of cardiac tissue is detected in the left ventricular cardiac region of the heart by the first cardiac signal. In one embodiment, the coupling interval time period is started once the contractions of cardiac tissue sensed in the first cardiac signal exceeds a predetermined threshold value.
In an additional embodiment for treating ventricular fibrillation, the defibrillator system monitors the first cardiac signal across the first cardiac region and a second cardiac signal across a second cardiac region. In one embodiment, the first cardiac region is a left ventricular cardiac region of the heart and the second cardiac region is a right ventricular cardiac region of the heart. Upon detecting a ventricular fibrillation, the defibrillator system delivers defibrillation shocks during the occurrence of both a coupling interval time period started once a contraction of cardiac tissue is detected in the left ventricular cardiac region of the heart and an up-slope portion of a coarse arrhythmia complex detected in the right ventricular cardiac region of the heart. Coarse ventricular fibrillation complexes are large amplitude cardiac electrogram signals detected during a ventricular fibrillation that in display regular periodic electrogram wave structures.
In an additional embodiment, the defibrillator system counts the coarse ventricular fibrillation complexes detected in the second cardiac signal. Defibrillation shocks are then coordinated with the up-slope portion of an nth counted coarse ventricular fibrillation complex having an amplitude greater than a coarse complex threshold value. The coarse complex threshold value is based on a Standard Amplitude Morphology (SAM) value. A SAM value is an average ventricular contraction signal which is calculated from a predetermined number of the largest second cardiac signal peak-to-peak values detected over a predetermined time interval. In one embodiment the coarse complex threshold value is 50% of the calculated SAM value.
Additionally, the delivery of the defibrillation shock is coordinated with a coupling interval time period, which is started once a contraction of cardiac tissue sensed in the first cardiac signal exceeds the predetermined threshold value. Upon detecting such a signal, the defibrillator system starts a coupling interval timer which counts off the predetermined coupling interval time period. In one embodiment, the delivery of the defibrillation shock is coordinated to occur during the coupling interval time period for the first cardiac signal and the up-slope portion of the nth counted coarse ventricular fibrillation complex of the second signal having an amplitude greater than the coarse complex threshold value. In this way the defibrillation shock may be coordinated with a ventricular condition from the first cardiac signal and/or the up-slope portion of a ventricular fibrillation complex from the second cardiac signal.